Reusable launch vehicles (RLVs), such as the space shuttle, repeatedly travel into or beyond the earth's upper atmosphere and then return to the earth's surface. During each mission, the RLVs experience extreme temperatures, ranging from −250° F. while in orbit to more than 2000° F. upon re-entry into the Earth's atmosphere. Certain areas of the RLV, such as the Wing Leading Edge (WLE), experience localized heating that can exceed 3000° F. Because of the extreme temperatures, the vehicle and its contents must be protected by a thermal protection system (TPS). The TPS is an outer covering of insulation, the purpose of which is to prevent the inner load-carrying structure of the vehicle from reaching/exceeding a certain maximum temperature. For the space shuttle the temperature limit is set by the lightweight aluminum structure of the shuttle, which begins to weaken at temperatures above 350° F.
Thermal protection systems for RLVs consist of a large number, usually around 20,000, of insulative tiles. The tiles function to insulate the vehicle from the environment and to radiate heat away from the vehicle. In addition to protecting the vehicle from environmental heat sources, the insulative tiles also provide protection from localized heating from such sources as the vehicle's main engines, rocket boosters, and directional thrusters.
The current state-of-the-art insulative tile material used on the orbiter fleet is Lockheed's LI-900™ material, an all silica rigid fibrous tile material. The LI-900™ material has outstanding thermal heat transfer characteristics and has a low coefficient of thermal expansion (CTE). However, it has a number of drawbacks such as low strength and relatively low thermal stability, which leads to sintering and shrinkage upon high thermal exposure. It is used only in areas of the TPS where temperatures do not exceed 2400° F. All areas exposed to a higher temperature are fitted with high temperature composites, which are very high in cost.
To overcome the drawbacks associated with LI-900™ tiles and to expand the use of economic tile TPS, alumina enhanced thermal barrier (AETB) was developed. AETB is a rigid, three-component tile material comprising silica fibers, alumina fibers, and aluminoborosilicate fibers. AETB is a preferred insulative material because it exhibits higher strength, higher thermal stability, and higher sintering and shrinkage resistance than LI-900™ tile. However, the AETB suffers from high thermal heat transfer and a large coefficient of thermal expansion. The greatest advantage of AETB is its ability to receive reaction cured glass (RCG) and toughened unipiece fibrous insulation (TUFI) coatings, which greatly increases its surface strength making it much easier to handle and gives it much improved impact resistance. However, its relative high heat transfer rate prevents its widespread use.
The shortcomings in LI-900™ tile and AETB tile led to the development of yet another insulative tile material: Boeing Rigid Insulation™ (BRI). Like AETB material, BRI™ material is a combination of silica and alumina fibers. BRI™ tile, however, includes boron carbide, which allows for more effective bonding of the fibers of the insulation. Further, BRI tile is produced in a manner which orients the fibers in a plane roughly perpendicular to the flow of heat through the tile, resulting in a tile with higher strength than LI-900™ tile and better thermal heat transfer properties than AETB tile.
Both AETB and BRI™ tiles are extremely porous, containing between 90% and 96% of void space per volume of material. The tremendous amount of void space within the insulation is responsible for the observed low thermal conduction of the tiles. However, the low density of the tiles is a double edged sword. Such tiles contain so much empty space that a high degree of heat may be allowed to move through the insulation in the form of radiation, lowering the effectiveness of the low density insulative tile. The radiation is of significance, since radiant heat transfer is the predominant heat transfer mechanism at temperatures above about 1000° F.
To further improve the thermal insulating properties of ceramic fiber insulation materials, a method is needed that enhances the thermal properties of silica and alumina fiber insulation by reducing the ability of heat to radiate through such a material. What is further needed is a method of reducing the ability of heat to radiate through the insulation without adversely affecting the mechanical properties of the insulation or adding substantially to the weight of the insulation.